Gas turbine control systems are well known in the art. These control systems are specially configured for use with particular civilian or military aircraft application For example, a fighter aircraft must be capable of undergoing violent maneuvers which require changes in engine thrust to severely accelerate or decelerate the aircraft. The fact that the aircraft must be able to perform severe maneuvers dictates the configuration of the engine control system.
An example of an engine used in fighter aircraft is the F100 engine manufactured by Pratt and Whitney Aircraft, a division of The United Technologies Corporation, the assignee of the present application. The F100 engine is a multiple spool, axial flow turbine power plant having a mixed flow, afterburning turbo fan engine configuration. The engine is characterized by a fan or low compressor coaxial with a high compressor rotor. Both the fan and high compressor have vanes whose angles are adjustable while the rotor blades are moving. The engine also has a variable area exhaust nozzle and an afterburner.
As is well known, gas turbine jet engines are most efficiently operated at high values of engine pressure ratio (EPR) as a function of airflow (W), which values can be raised by reducing the exhaust nozzle cross-sectional area. However, too high an engine pressure ratio for a given rate of airflow will produce engine stall. An engine stall condition occurs for every value of airflow for a given engine pressure ratio and therefore defines a stall region bounded by a stall line in a graph of engine pressure ratio versus engine airflow. The allowable engine pressure ratios for a given airflow defines an operational line displaced away from the stall line. Engine control systems for aircraft designed to have these extreme performance characteristics must be specially configured to ensure that the engine can never reach a stall condition, irrespective of the aircraft's current maneuver or the operation of the engine's afterburner.
Consequently, the operating line (i.e. the selected values of engine pressure ratio (EPR) as a function of airflow) is deliberately depressed to avoid the engine stall region. The depressed operating line also corresponds to a regime of lower engine performance and is therefore undesirable. The more maneuverable the aircraft, the more margin there must be in the operating line for the aircraft to unequivocally avoid the stall region, since the stall line can be effected by the in-flight maneuvers of the aircraft. The increased stall margin required by high performance aircraft further lowers the operating line and hence the efficiency at which the control system operates the engine. Not only is the engine efficiency reduced but operating at the depressed operating line results in higher turbine temperatures and correspondingly reduced engine life.
This stall margin must be programmed into the control system even though the aircraft will spend relatively little time operating at the outer reaches of the aircraft's performance envelope. In the past, especially with the HIDEC (Highly Integrated Digital Electronic Control) program, there have been efforts to create engine control systems which would allow the aircraft to work at a higher operating line when the aircraft is, for example, at sub-sonic speeds and flying straight and level. This would enable the engine to operate more efficiently. The HIDEC control system simply adjusted the engine pressure ratio (EPR) upwards in value when the pilot manually indicated to the control that the aircraft was engaged in straightforward non-violent maneuver. However, simply raising the engine pressure ratio significantly raises the turbine inlet temperature and decreases engine life.
It would be advantageous to have control system for a gas turbine engine which can selectively program the engine to operate with reduced stall margin at a selected value of thrust, yielding higher efficiency and longer engine life. The present control system is drawn towards such an invention.